Use STSADM mergecontentdbs. Outlined here,
www.toddklindt.com/.../Post.aspx?ID=53

That's so funny. I was just getting ready to respond with an article on that
very cmdlet

I've had mixed results using MergeContentDB myself, that is why I use the
script

And I've had problems with the manual method which is why I use
mergecontentdbs.

In SharePoint 2003, and I think 2007 it was possible to make a backup
that was successful that you could not restore. Of course you didn't
find this out until after you had deleted the original instance of the
site collection. I got bit by that twice. It was not fun.

Here is the batch file I use to move site collections between DBs-

stsadm -o preparetomove -site %1

stsadm -o setsitelock -url %1 -lock readonly

stsadm -o backup -url %1 -filename d:\backups\%2.bak -overwrite

stsadm -o setsitelock -url %1 -lock none

pause

stsadm -o deletesite -url %1

stsadm -o restore -url %1 -filename d:\backups\%2.bak

stsadm -o setsitelock -url %1 -lock none

stsadm -o preparetomove -site %1 -undo



Put that in a text file and change the name to something like MoveSC.bat

Usage:
MoveSC.bat <Url of SC> <Name for backup file>
ie MoveSC http://Sharepoint/sites/foo foo_backup

The pause is in there so you can confirm the backup succeeded before
deleting the site.

To ensure it goes to the DB you want, set all the other DBs to Offline, and
the one you want to receive the site to Ready. (Central Admin -> Application
Management -> Content Databases -> Web Application in Question Click on DB
name and set DB status.

Note: Setting a DB to Offline does not take it 'offline', it does have two
effects: 1) no new sites can be created in it; 2) It will not sync with AD
via the SSP.